I don't think you can generalize about the area like this. The southern part
of the area Ken named was already becoming Frankish at this time, though
there was not yet a Frankish empire.

But Frisia was still not under Frankish power until the time of Charlemagne
I believe, around 800.

"My best guess is that the Belgium area was now more influenced by French"

That is a confusing statement for this period. A big part of what became
Belgium was at this time the core of the developing Salian nation, which
would become the core of the Frankish empire. Try Googling Toxandria. So
this area was breaking off from Gaul, and in turn starting to influence
Gaul.

Coming back to Ken's question, the Toxandrian area in the southeast of the
area Ken named had been taking immigrants from the east, modern Germany,
since before Caesar. The North, for example Frisia, and the coast, including
the coast of Belgium, had been receiving coast hopping Germanic from the
direction of Scandinavia since some point during the Roman empire.

Concerning the admixture between Gaulo-Romans and newcomers, this would of
course partly depend on birthrates of the two groups.

The area was aleady populated by many people that were an amalgen of
Jutes , Angles-Saxons , which people were of Frisian and North Germanic
tribes and was pretty much under the umbrella of the Frankish Empire. It was
about at this time , that these same people moved onward into SouthEast
England.

There doesn't seem to be a known replacement into the main homeland of the
many Belgic tribes . The Franks thought they did ,but didn't really have a
good hold on these Gaulic/Celtic belicose tribes . Thus with the
Christianization of the Franks , these non -Christians must have decided to
get out of town . My best guess is that the Belgium area was now more
influenced by French , Brittany ,on one part and Nordic immigres on the
other part . Thus the Walloons and Belgics , starting in the 800s onward